Electrical apparatus containing a graft polyethylene terephthalate-polystyrene insulating film to be used together with an insulating oil

ABSTRACT

An electrical apparatus comprising a conductor, an insulating layer covering said conductor which consists of an insulating film, and an insulating oil impregnated in said layer, characterized in that said insulating film consists of polyethylene terephthalate and styrene polymer grafted to said polyethylene terephthalate, an amount of styrene polymer being 10 to 200 percent by weight of said polyethylene terephthalate.

United States Patent [151 3,657,468 Tsukioka et al. [451 Apr. 18, 1972 [54] ELECTRICAL APPARATUS [56] References Cited CONTAINING A GRAFT POLY- if V UNITED STATES PATENTS POLYSTRENE INSULATING FILM o mag: BE USED TOGETHER WITH AN 3:252:880 5/1966 M26113... 11111204/159115 INSULATING OIL 2,764,718 9/ 1956 Peck et al. ..260/873 72 Inventors: Hideo Tsukioka; Nobuhiko Shito, both of 2,876,187 3/ 1959 Wolmskl Hitachi, Japan 3,054,770 9/1962 Rogers ..174/121.4 3,374,114 3/1968 Wiener ..l74/l10 SR [73] Asslgneez H1tach1, Ltd., Tokyo, Japan 22 Filed; 20 97 Primary Examiner-William H. Short Assistant Examiner-Edward Woodberry 1 1 pp N94 651362 Attorney-Craig, Antonelli, Stewart & Hill Related US. Application Data 57] ABSTRACT [63] fgg fi gggggg of May An electrical apparatus comprisingaconductor, an insulating layer covering said conductor which consists of an insulating film, and an insulating oil impregnated in said layer, charac- [30] Fomgn Apphcauon Pnomy Data terized in that said insulating film consists of polyethylene June 8, 1966 Japan ..41/36506 terephthalate and styrene polymer grafted to said polyethylene terephthalate, an amount of styrene polymer [52] 11.8. C1 ..174/25, 174/110 PM, 174/1 10 SY, being 10 to 200 percent by weight of said polyethylene 204/ 159.15, 252/632, 260/873 terephthalate. [51] Int. Cl. ..C08g 39/10, HOlb 3/42, HOlb 7/00,

- HOlb 17/34 3 Claims, 4 Drawing Figures [58] Field of Search ..l74/25 R, 26 R, 120, 121 R,

174/121 SR, 125, 110 MP, 110 SR, 110 R; 252/632; 260/873; 204/159.15, 159.19

Patented April 18, 1972 3,65 7,468

INVJENTORS H EO TSLLK! OKFI NOBlM-HKO SHFVO ATTORNE S ELECTRICAL APPARATUS CONTAINING GRAFI POLYETHYLENE TEREPI-ITHALATE-POLYSTRENE INSULATING FILM TO BE USED TOGETHER WITH AN INSULATING OIL This application is Continuation-ln-Part Application of U.S. Ser. No. 639,882 filed on May 19, 1967 and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to an electric apparatus containing an insulating film and an insulating oil and also relates to a process for preparing said insulating film.

In a large-size electric apparatus, such as, for example, a large-capacity transformer and an oil-impregnated cable for transmission line, a conductor is covered by an insulating layer containing an insulating oil. When a resin insulating material is used as a material constituting the insulating layer together with such insulating oil, it is necessary for the resin insulating material to have both excellent oil resistance and high dielectric breakdown strength in the insulating oil.

In spite of the fact that the thermoplastic resin insulatingv materials have essentially high dielectric breakdown strength, their dielectric breakdown strength, especially impulse dielec tric breakdown strength, in the insulating oil is not so high and sometimes it is lower than that of oil-impregnated insulating paper which has been heretofore used. The oil resistance of a thermoplastic resin insulating material having high dielectric breakdown strength in the insulating oil is generally poor. On

the other hand, the dielectric breakdown strength in an insu lating oil is generally low. Among other thermoplastic resins, polyester resin, particularly, polyethylene terephthalate has an excellentoil resistance but its breakdown strength is rather poor in the insulating oil.

In electric apparatuses wherein insulation has been carried out with a combination of an insulating oil and a resin insulating material, alternating current dielectric breakdown or impulse dielectric breakdown often starts from an insulating layer containing the insulating oil. If breakdown is prevented on the surface or at the inside of the resin insulating material adjacent to the insulating oil, therefore, dielectric breakdown strength of the whole insulating layer will-be improved.

It is an object of the present invention to provide an electric apparatus including an insulating layer covering a conductor and an insulating oil impregnated in the insulating layer.

It is another object to provide an electric apparatus includin ga resin insulating material having both improved electrical and oil resistance.

It is still another object to provide a resin insulating material which is not injuriously affected by an insulating oil and can preserve excellent breakdown strength.

These and other objects and advantages of the present invention will become apparent to those skilled in the art from the following description and appended claims.

SUMMARY OF THE INVENTION We have attempted the use of various combinations of two or more thermoplastic resins such as polyethylene film, polypropylene film, polycarbonate film, polyethylene terephthalate film, polyamide film and polystyrene film and have examined their dielectric breakdown strength in insulating oils. As a result, it has been found that particularly excellent breakdown strength in the oils is obtained by a combination of polystyrene film and another thermoplastic resin. Such a combination, however, can not be put to practical use in electrical apparatuses because the oil resistance of polystyrene film is extremely poor. Such has been the case also with combinations of polystyrene and another thermoplastic resin having excellent oil resistance.

As a result of further researches, we have completed the present invention. According to the present invention, a resin insulating material to be used together with an insulating oil which preserves the original properties of a thermoplastic resin insulating material and has excellent dielectric breakbe obtained by graft-copolymerizing one or more styrenes represented by a general formula wherein X is selected from the group consisting of a hydrogen atom, halogen atoms and alkyl groups and Y, to I, are selected. from the group consisting of a hydrogen atom, halogen atoms, alkyl groups and a methoxy group to part or all of a thermoplastic resin insulating material. Thus, resin insulating materials obtained by graft-copolymerizing said styrenes to thermoplastic resin insulating materials having excellent oil resistance show such excellent oil resistance that it cannot be obtained by polystyrenes and also show excellent breakdown strength in insulating oils similar to that of polystyrenes'themselves.

The thermoplastic resin insulating materials which may be used in the present invention is polyethylene terephthalate.

These resin insulating materials may be used in the form of, for example, film, plate or tube. Alternatively, they may be used in any form adapted to electrical apparatuses. Further, they may be generally formed articles or may consist of woven fabric or non-woven fabric.

The styrenes represented by the above-mentioned general formula which may be graft-copolymerized to said thermoplastic resin insulating materials in the present invention include, for example,

styrene, CH-CH 2,5-dichlora- CH=CH;,

l styrene,

4-chlorostyrene, CH=CHz, Z-methyl- CH=CH,

l styrene,

OCH: l Cl 2,6-dimethyl- CH=CH;, 4-ethylstyrene, CH=CH:,

styrene, l

I OH; H3C

4-propylstyrene, CH=OH a-methoxy- CH=CH1,

styrene, l OOCH:

a-methyl- C(CH;)=CH a-methyl-(i- C(CH:)=CH

styrene, chlorostyrene,

a-methyl-Z- C(CHa)=CH1, etc. methyl-3- ehlorostyrene,

In the present invention, said styrenes may be graftcopolymerized to thermoplastic resin insulating materials according to either of vapor phase polymerization process or liquid phase polymerization process. Alternatively, the styrenes may be applied to part or all of the thermoplastic resin insulating material and then the whole may be heated. When two or more styrenes are graft-copolymerized, they may be copolymerized simultaneously or stepwise.

In the present invention, the larger the amount of polystyrene grafted may be, the higher breakdown strength in the insulating oil is obtained. The amount of polystyrenes grafted is preferably 10 to 200 percent by weight of polyethylene terephthalate. When less than 10 percent, sufficient breakdown strength is not attained because the amount of polystyrene is too small. When more than 200 percent, satisfactory mechanical properties, especially, elongation and strength cannot be obtained. Especially, where the graft amount is from about 40 to 100 percent by weight, most excellent results can be expected.

The resin insulating material obtained by the present invention may be employed, for example, in the insulation of coil or in the insulation of cable as shown in the accompanying drawings in which:

FIGS. 1 to 3 are partial sectional sketches of an insulation structure for coil of a large-size transformer according to the present invention and FIG. 4 is a partial sectional sketch of an insulation structure for a high-tension insulated cable according to the present invention.

With reference to FIGS. 1 to 3, (1) is a conductor, (2), (3) and (4) are insulating material film layers, and (5) and (6) are thermoplastic resin insulating material layers having a graftcopolymer layer obtained by the present invention. The whole is impregnated with an insulating oil. The resininsulating material layers of the present invention (5) and (6) may be optionally combined with other insulating material layers (2), (3) and (4) and may be applied as two or more separate layers.

With reference to FIG. 4, (l) is a cable conductor; (2) is a resin insulating material layer obtained by the present invention; (3) is a layer of another insulating material; and (4) is an aluminium sheath. The inside of the insulating layers is impregnated with an insulating oil also in this case. The resin insulating material of the present invention (2) may be applied as two or more separate layers also in this insulating cable.

EXAMPLES OF THE INVENTION The following examples and comparative examples are illustrative only of the present invention and should not be construed as limiting the scope thereof, which is properly delineated in the appended claims.

EXAMPLE I As a thermoplastic resin insulating material a polyethylene terephthalate film was used which had been exposed to 2 percent ozone for 1 to 2 hours. Styrene which had been sufficiently degased by repeating freezing, exhaust and melting thereof three times at a reduced pressure of 10" to 10' mm.I-Ig was used as a styrene. These materials were subjected to vapor phase graft-polymerization in an evacuated vessel at 90 C. for 14 hours. The resultant resin insulating material increased by 48 percent in its weight and 79 percent in its thickness. The whole thickness of the product was 0.09 mm.

Insulating layers were applied onto a conductor by the construction shown in FIGS. 1 to 3, using a combination of the abovementioned resin insulating material and a polyethylene terephthalate film having a thickness of 0.05 mm. In the above-mentioned insulating layers each material was applied by half-covered wrapping. Thus one layer of the polyethylene terephthalate film ((2) in the Figures) was first applied onto the conductor. Onto the film layer, one layer of said graft film ((5) in the Figures), five layers of the polyethylene terephthalate film ((3) in the Figures), one layer of the graft film ((6) in the Figures) and one layer of the polyethylene terephthalate film ((4) in the Figures) were applied in order mentioned. The above-mentioned insulating layers were also impregnated with an insulating oil (JIS No. 2 grade).

EXAMPLE 2 I Methylstyrene was used as a styrene and a styrene graft resin insulating material was obtained in the same manner as in Example 1. The resultant resin insulating material increased by 41 percent in its weight and 62 percent in its thickness. The whole thickness of the product was 0.081 mm.

Insulating layers were applied onto a conductor by the same construction as in Example I, using a combination of the above-mentioned resin insulating material and a polyethylene terephthalate film.

EXAMPLE 3 A polyethylene terephthalate film having a thickness of 0.05 mm. and the styrene graft film obtained in Example I were used. Each material was applied onto a cable conductor by butt wrapping. Thus one layer of the polyethylene terephthalate film, one layer of the styrene graft film, 15 layers of the polyethylene terephthalate film, one layer of the styrene graft film and one layer of the polyethylene terephthalate film were applied in order mentioned. Finally an aluminium sheath was provided on the most outer side.

EXAMPLE 4 A polyethylene terephthalate film having a thickness of 0.05 mm. and the methylstyrene graft film obtained in Example 2 were used. Insulating layers were applied onto a cable conductor by the same construction as in Example 3 and an aluminium sheath was provided.

COMPARATIVE EXAMPLE 1 Onto a conductor (coil) shown in Example 1, nine layers of a polyethylene terephthalate film alone were applied by halfcovered wrapping. The whole was then impregnated with an insulating oil.

COMPARATIVE EXAMPLE 2 Onto the same cable conductor as in Example 3, 20 layers of a polyethylene terephthalate alone were applied by butt wrapping. An aluminium sheath was provided on the layers and the whole was then impregnated with an insulating oil.

The dielectric breakdown strength of the insulated coils and insulated cables obtained in the above-mentioned Examples and comparative Examples is shown in Table 1.

TABLE 1 Whole Thickness AC breakdown voltage; Impulse breakdown thickness of styrene crest value (KV) voltage (KV) of graft layer insulating (mm/one Item layer (mm.) side) Avg. Max. Min. Avg. Max. Min.

Sample:

Example:

l 1. 9 0. 019 184 201 177 256 273 242 *1. 9 0. 015 177 195 I68 219 241 I87 Comparative Example 1.. 1.8 161) 177 153 180 201 170 Example:

1. 0 0. 019 I38 155 121 170 197 149 4 1. 0 0.015 128 149 109 145 183 111 Comparative Example 2 1.0 110 104 115 128 103 The tl lckness o l' the insulating layer between two coll conductors (1).

EXAMPLE 8 An insulating cable was produced by the same construction as in Example 3 using the resin film obtained in Example 6. 5 The dielectric breakdown strength of the insulated coils and cables obtained in Examples 5 to 8 is shown in Table 3.

TABLE 2 Dimen- Change of Change sional Oil tensile oi elon- Sample change absorption Solubility strength gation Polystyrene film Measurement was impossible because the sample was completely dissolved Polyethylene terephthalate film. No change. Not absorbed. Not dissolved. N change. N 0 change Graft film obtained in Ex. 1 do Slightly absorbed do .d o. Graft film obtained in Ex. 2 do "do do "do Do.

TABLE 3 Whole Thickness AC breakdown thickness of styrene voltage; crest value Impulse breakdown of insulatgraft layer (KV) voltage (KV) ing layer (mm./0ne 4 (mm) side) Avg. Max. Mm Avg. Max. Min.

The above table shows the oil resistance of each sample measured by allowing it to stand in an insulating oil at 95 C. for one week.

As is clear from the above description, the resin insulating materials obtained according to the present invention have both excellent dielectric breakdown strength and oil resistance. As a result of our further researches, we have confirmed a new fact that more excellent dielectric breakdown strength in an oil can be obtained by heating and pressing the resin insulating material obtained according to the present invention by means of a roller. The reason why the dielectric breakdown strength can be improved is supposed to be as follows: Thermoplastic resin insulating materials to which said styrenes have been graft-copolymerized have many minute holes at their polymerized part and corona discharge is likely to arise which may reduce dielectric breakdown strength, but said many holes are crushed by the heating and pressing and hereby harmful holes are removed.

The temperature and pressure in said heating and pressing of the present invention are not within a specific range. The temperature and pressure may merely be required to be sufficient to crush minute holes present at the graft-copolymerized part of the resin.

The following examples illustrate the above-mentioned operation.

EXAMPLE 5 The graft film obtained in Example 1 was passed through heated rollers five times at a temperature of 95 C. to 105 C. and a pressure of about 100 kg./cm. An insulated coil was then produced by the same construction as in Example 1 using the above-mentioned resin film.

EXAMPLE 6 The graft film obtained in Example 2 was heated and pressed under the same condition as in Example 5. An insulated coil was produced by the same construction as in Example 1 using the above-mentioned resin film.

EXAMPLE 7 An insulated cable was produced by the same construction as in Example 3 using the resin film obtained in Example 5.

As is clear from the comparison between Tables 1 and 3, heat and pressure treatment further increases dielectric breakdown strength in an oil.

As described above in detail, it was impossible to obtain both excellent dielectric breakdown strength and excellent oil resistance simultaneously in a mere cemented or glued article of a previous thermoplastic resin having excellent dielectric breakdown strength and a thermoplastic resin having excellent oil resistance, but the desired object could be attained by graft-copolymerizing a styrene to a thermoplastic resin having excellent oil resistance according to the present invention.

What we claim is:

1. An electric apparatus comprising a conductor, an insulating layer covering said conductor which includes an insulating film of thermoplastic resin, and an insulating oil impregnated in said layer, characterized in that said insulating film consists of polyethylene terephthalate and polystyrene grafted thereto in an amount of 10 to 200 percent by weight of said polyethylene terephthalate, wherein said polystyrene is obtained by graft-polymerizing styrene compounds of the general formula 

2. An electric apparatus according to claim 1, in which said amount is 40 to 100 percent by weight.
 3. An electric apparatus according to claim 1, in which said polystyrene is derived from a compound selected from the group consisting of styrene, 2,5-dichlorostyrene, 4-chlorostyrene, 2-methylstyrene, 2,5-dichlorostyrene, 4-ethylstyrene, 4-propylstyrene, 3-methoxystyrene, Alpha -methylstyrene, Alpha -methyl-3-chlorostyrene and Alpha -methyl-2-methyl-3-chlorostyrene. 